How some Extremely Smart Hackers Pulled off the Most Audacious Cell-network Break-in

The base station's activities are governed by a base station controller, a special-purpose computer within the station that allocates radio channels and helps coordinate handovers between the transceivers under its control.

This controller in turn communicates with a mobile switching center that takes phone calls and connects them to call recipients within the same switching center, other switching centers within the company, or special exchanges that act as gateways to foreign networks, routing calls to other telephone networks (mobile or landline). The mobile switching centers are particularly important to the Athens affair because they hosted the rogue phone-tapping software, and it is there that the eavesdropping originated. They were the logical choice, because they are at the heart of the network; the intruders needed to take over only a few of them in order to carry out their attack.

A central processor coordinates the switch's operations and directs the switch to set up a speech or data path from one phone to another and then routes a call through it. Logs of network activity and billing records are stored on disk by a separate unit, called a management processor.

The key to understanding the hack at the heart of the Athens affair is knowing how the Ericsson AXE allows lawful intercepts—what are popularly called “wiretaps.” In AXE exchanges a remote-control equipment subsystem, or RES, carries out the phone tap by monitoring the speech and data streams of switched calls. It is a software subsystem typically used for setting up wiretaps, which only law officers are supposed to have access to. When the wiretapped phone makes a call, the RES copies the conversation into a second data stream and diverts that copy to a phone line used by law enforcement officials.

Ericsson optionally provides an interception management system (IMS), through which lawful call intercepts are set up and managed. When a court order is presented to the phone company, its operators initiate an intercept by filling out a dialog box in the IMS software. The optional IMS in the operator interface and the RES in the exchange each contain a list of wiretaps: wiretap requests in the case of the IMS, actual taps in the RES. Only IMS-initiated wiretaps should be active in the RES, so a wiretap in the RES without a request for a tap in the IMS is a pretty good indicator that an unauthorized tap has occurred.

It turns out Vodafone had not purchased the lawful intercept option at the time of the illegal wiretaps. The Vodafone system contained the software code necessary to intercept calls using the RES, even though it lacked the high-level user interface in the IMS normally used to facilitate such intercepts.

That odd circumstance would turn out to play a role in letting the Athens hackers illegally listen in on calls and yet escape detection for months and months.

It took guile and some serious programming chops to manipulate the lawful call-intercept functions in Vodafone's mobile switching centers. The intruders' task was particularly complicated because they needed to install and operate the wiretapping software on the exchanges without being detected by Vodafone or Ericsson system administrators. From time to time the intruders needed access to the rogue software to update the lists of monitored numbers and shadow phones. These activities had to be kept off all logs, while the software itself had to be invisible to the system administrators conducting routine maintenance activities. The intruders achieved all these objectives.

They took advantage of the fact that the AXE allows new software to be installed without rebooting the system, an important feature when any interruption would disconnect phone calls, lose text messages, and render emergency services unreachable. ..